fanleaf degeneration, sometimes called fanleaf virus, one of the oldest known virus diseases affecting vines. Records of it date back some 200 years in Europe and there are indications that it may have existed in the Mediterranean and Near East since grape culture began. Rather than being a single disease, it is in fact a complex of related diseases which include forms known as yellow mosaic and veinbanding. Shoot growth is typically malformed, leaves are distorted and asymmetric, and teeth along the edge are elongated. Shoots show abnormal branching with double nodes (see fasciation), short internodes, and zigzag growth. Leaves on infected plants look fanlike—hence the name. Bunches are smaller than normal, with poor fruit set and many shot berries. Sensitive varieties such as Cabernet Sauvignon can lose up to 80% of potential yield and have a shortened productive life.
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